1. Field of the Invention
This invention relates to a thermographic photographic element and more particularly, to a photothermographic element having a high sensitivity and which experiences a minimal change of sensitivity under varying conditions during storage.
2. Prior Art
From the contemporary standpoints of environmental protection and space saving, it is strongly desired in the medical imaging field to reduce the quantity of spent solution. Needed in this regard is a technology relating to thermographic photosensitive materials for use in medical diagnosis and general photography which can be effectively exposed by means of laser image setters and laser imagers and produce clear black images having a high resolution and sharpness. These thermographic photosensitive materials offer to the customer a simple thermographic system which eliminates the need for solution type chemical agents and is not detrimental to the environment.
On the other hand, the recent rapid progress of semiconductor laser technology has made it possible to reduce the size of medical image output devices. As a matter of course, there were developed techniques relating to infrared-sensitive photothermal silver halide photographic material which can utilize a laser diode as a light source. The spectral sensitization technique is disclosed, for example, in JP-B 10391/1991 and 52387/1994, JP-A 341432/1993, 194781/1994, and 301141/1994. The antihalation technique is disclosed, for example, in JP-A 13295/1995 and U.S. Pat. No. 5,380,635. Since the infrared exposure system permits the visible light absorption of sensitizing dyes and antihalation dyes to be considerably reduced, a substantially colorless photosensitive material can be readily produced.
A combination of the thermographic technology with the infrared exposure technology enables a photosensitive material which eliminates a need for liquid.
Since spectral sensitizing dyes capable of absorbing infrared radiation, however, generally have a high reducing power due to a high HOMO (highest occupied molecular orbital), they tend to reduce silver ions in photosensitive materials exacerbating the fog thereof. In particular, these photosensitive materials experience a substantial change of performance during storage under hot humid conditions or long-term storage. If dyes having a low HOMO are used for preventing the photosensitive material from deteriorating during storage, spectral sensitization efficiency and sensitivity become low because their LUMO (lowest unoccupied molecular orbital) is relatively low. These problems relating to sensitivity, storage stability, and performance change arise not only with wet photographic photosensitive materials, but more outstandingly with photothermographic materials.
The supersensitization technique has been developed for overcoming such infrared sensitization problems. Known infrared supersensitizers for use in thermographic systems include aminopolycarboxylic acid derivatives as disclosed in JP-A 4241/1990, and heteroaromatic mercapto compounds and heteroaromatic disulfide compounds as disclosed in JP-A 182639/1992 and 341432/1993. The aminopolycarboxylic acid derivatives provide weak supersensitization effect and low sensitivity whereas the heteroaromatic mercapto and disulfide compounds allow the sensitivity to vary during storage under hot humid conditions.
In the printing field, image forming systems exhibiting photographic characteristics including ultrahigh contrast (especially gamma values of 10 or higher) are available in order to improve the reproduction of continuous tone images or halftone images or the reproduction of line images. From the standpoints of environmental protection and space saving, it is strongly desired to reduce the amount of waste solution. Recent research efforts achieved noticeable reduction of waste solution. In systems using processing solutions of chemicals, it is impossible to eliminate the waste solution. Accordingly, the printing field waits for the practical implement of the thermographic system which is simple and ecologically safe since it eliminates the use of processing solutions of chemicals.
There have been proposed image forming processes using photothermographic materials entailing a developing step by heat treatment. Such materials are disclosed, for example, in JP-B 4924/1968 and 6582/1969, JP-A 6074/1971, 97523/1973, and 2781/1995, and U.S. Pat. No. 5,468,603. These photothermographic materials, however, are not suited for the manufacture of printing plates because of low gamma or soft gradation.
In the printing field, photographic characteristics ensuring ultrahigh contrast are desired as described above. The desired ultrahigh contrast is accomplished using hydrazine derivatives as disclosed in U.S. Pat. No. 5,496,695. Where hydrazine derivatives are used, however, stable images are not obtainable since the sensitivity largely changes with changes of temperature and time of heat development. An improvement in this regard is desired.
Fog by heat development is also a crucial problem. A number of proposals have been made for reducing the fog of thermographic silver halide photosensitive materials. For example, U.S. Pat. No. 3,589,903 discloses mercury salts. There are also known carboxylic acids such as benzoic acid and phthalic acid from U.S. Pat. No. 4,152,160; benzoylbenzene acid compounds from U.S. Pat. No. 4,784,939; indane and tetralin carboxylic acids from U.S. Pat. No. 4,569,906; dicarboxylic acids from U.S. Pat. No. 4,820,617; heteroaromatic carboxylic acids from U.S. Pat. No. 4,626,500; halogenated compounds from U.S. Pat. Nos. 4,546,075, 4,756,999, 4,452,885, 3,874,946 and 3,955,982; halogen molecules or heterocycles associated with halogen atoms from U.S. Pat. No. 5,028,523; palladium compounds from U.S. Pat. No. 4,103,312 and GB 1,502,670; iron group metals from U.S. Pat. No. 4,128,428; substituted triazoles from U.S. Pat. Nos. 4,123,374, 4,129,557 and 4,125,430; sulfur compounds from U.S. Pat. Nos. 4,213,784, 4,245,033 and JP-A 26019/1976; thiouracils from U.S. Pat. No. 4,002,479; sulfinic acids from JP-A 123331/1975; metal salts of thiosulfonic acid from U.S. Pat. Nos. 4,125,403, 4,152,160 and 4,307,187; combinations of metal salts of thiosulfonic acid with sulfinic acid from JP-A 20923/1978 and 19825/1978; and thiosulfonates from JP-B 50810/1987, JP-A 209797/1995 and 43760/1997. Also, JP-A 42529/1976 and JP-B 37368/1988 discloses disulfide compounds. None of these patents describe whether or not these compounds are effective for suppressing changes under different heat development conditions of the sensitivity of ultrahigh contrast photosensitive material systems using ultrahigh contrast enhancers.
JP-B 21925/1994 discloses the use of hydrazine derivatives and disulfides in photographic silver halide materials which are developed with developer solutions. This patent describes that the addition of disulfides is effective for improving the stability of photographic properties during storage of the material, but refers nowhere to the effect of such compounds in thermographic systems, for example, whether such compounds are effective for improving the heat development stability.